Fuel tanks can contain potentially combustible combinations of oxygen, fuel vapors, and ignition sources. In order to prevent combustion in aircraft fuel tanks, aviation regulations require actively managing the ullage of fuel tanks, such that the oxygen partial pressure in the ullage is less than 12%. On-Board Inert Gas Generation Systems commonly use bleed air and pressurized hollow fiber membranes to produce ODA for fuel tank ullages. In hollow fiber membranes, the diffusivity of nitrogen is less than the diffusivity of oxygen and water vapor. Hollow fiber membrane systems require pressurized air to drive the separation of nitrogen from oxygen and water vapor in an air stream. However, the pressure of bleed air extracted from an aircraft engine compressor varies throughout a mission. Bleed air pressure can be lowest when the demand for inert gas is highest. During descent, outside air fills the ullage due to the pressure differential between the tank and ambient air. The operating setpoint of an aircraft engine is driven by pneumatic loads during descent so that the cabin pressurization, environmental control, wing de-icing, and inerting systems have sufficient bleed air pressure. Furthermore, aircraft design is trending toward lower pressure bleed systems and increasingly electric power distribution architectures. Accordingly, the use of high pressure, hollow fiber membrane inerting systems can be problematic for these systems.
Relatedly, fire suppression systems, such as those deployed in aircraft cargo holds, use halogenated chemicals to prevent combustion and/or fire. Halogenated fire suppression agents can be safe for human exposure; however, they are known to be detrimental to the Earth's atmospheric ozone layer. Hypoxic air can also be used for fire prevention and suppression. If people or animals are exposed to the hypoxic air, the partial pressure of oxygen must be closely regulated such that the gas prevents ignition and suppresses combustion while simultaneously remaining suitable for respiration.